1. Field of the Invention
The invention relates to a method for forming a contact portion of a semiconductor device, and more particularly to a method for forming a contact portion for connecting devices provided in the semiconductor device with an outside wiring.
2. Description of the Related Art
Various semiconductor devices are provided in the dielectric layers with contact holes through which any devices provided in the silicon substrates are connected with outside wirings, and conductive material is buried in the contact hole to form a contact portion. A typically known technique related to this uses CVD method to deposit tungsten for the conductive material in the contact hole and causes the same to grow selectively and be buried therein.
The tungsten selective growth technique has such an advantage that a deposit layer can be buried in the contact hole with high aspect ratio (the ratio of height B and width A of the contact holes: B/A) simply and reliably without necessity of etching-back process or the like.
However, the tungsten selective growth technique has a problem of deteriorated properties of obtained semiconductor devices resulting from the fact that WF.sub.6, the material for CVD method, reacts with the surface of the silicon substrate and wastes Si in the midst of deposition of tungsten.
It has been proposed that polysilicon be substituted for tungsten to be deposited and buried in a contact hole by CVD method. Polysilicon is however hard to be selectively deposited and grown in the contact hole. Hence, a polysilicon layer is first formed on an area on the substrate including a contact hole and then etched back to allow only a deposit layer of the polysilicon in the contact hole to remain. The polysilicon deposit layer in the contact hole is doped with p-type or n-type impurity by ion implantation to be given predetermined conductivity.
The above method using polysilicon has such a problem in case of a contact hole of high aspect ratio that since height of the hole is larger, an impurity is not doped uniformly over the whole polysilicon deposit layer, resulting in that a resultant contact portion is higher in resistance.